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United States Patent |
5,223,544
|
Burkett
,   et al.
|
June 29, 1993
|
Process for the removal of foreign materials from a post-consumer
plyethylene terephthalate feed stream
Abstract
In today's environment, it is becoming increasingly more important to
recycle plastics including polyethylene terephthalate. Technology
available today allows for polyethylene terephthalate to be recycled into
a wide variety of useful products. For instance, carbonated beverage
bottles which are comprised of polyethylene terephthalate can be recycled
into new polyethylene terephthalate beverage bottles. However, a
relatively pure polyethylene terephthalate post consumer feed stream is
required in many such recycling programs. Unfortunately, such feed streams
are often contaminated with foreign material. This invention relates to a
process for removing foreign materials from a post-consumer polyethylene
terephthalate feed stream which comprises: (1) depolymerizing the
polyethylene terephthalate feed stream into a melt having a melt viscosity
which is within the range of 0.001 poise to 1000 poise, (2) feeding the
melt into a separation device, (3) allowing low density foreign materials
to float to the surface of the melt, (4) allowing high density foreign
materials to sink to the bottom of the melt, and (5) removing molten
polyethylene terephthalate oligomer from an area which is located
intermediately between the surface of the melt and the bottom of the melt.
Inventors:
|
Burkett; Eugene J. (Scott Depot, WV);
Jenks; Randy S. (Bidwell, OH)
|
Assignee:
|
Shell Oil Company (Houston, TX)
|
Appl. No.:
|
860963 |
Filed:
|
March 31, 1992 |
Current U.S. Class: |
521/48; 521/41; 521/48.5 |
Intern'l Class: |
C08J 011/06 |
Field of Search: |
521/48,48.5,41
|
References Cited
U.S. Patent Documents
3108082 | Oct., 1963 | Riehe et al. | 521/48.
|
3652466 | Mar., 1972 | Hittel et al. | 521/48.
|
3884850 | May., 1975 | Ostrowski | 521/48.
|
4143001 | Mar., 1979 | Raab et al. | 521/48.
|
4439550 | Mar., 1989 | Brennan | 521/48.
|
4542239 | Sep., 1985 | Lamparter et al. | 562/485.
|
4605762 | Aug., 1986 | Mandori | 562/485.
|
4609680 | Sep., 1986 | Fujita et al. | 521/48.
|
5045145 | Mar., 1992 | Rosen | 562/483.
|
5120768 | Jun., 1992 | Sisson | 521/48.
|
Foreign Patent Documents |
0019876 | Jun., 1973 | JP | 521/48.
|
9010667 | Sep., 1990 | WO | 521/48.
|
Primary Examiner: Michl; Paul R.
Assistant Examiner: Asinovsky; Olga
Claims
What is claimed is:
1. A process for removing foreign materials from a polyethylene
terephthalate feed stream which comprises: (1) depolymerizing the
polyethylene terephthalate feed stream into a liquid melt having a
viscosity which is within the range of 0.001 poise to 1000 poise, (2)
feeding the liquid melt into a separation device, (3) allowing low density
foreign materials to migrate to the surface of the liquid melt, (4)
allowing high density foreign materials to migrate to the bottom of the
liquid melt, and (5) removing depolymerized polyethylene terephthalate
from an area which is located intermediately between the surface of the
liquid melt and the bottom of the liquid melt.
2. A process as specified in claim 1 wherein the liquid melt has a
viscosity which is within the range of 0.01 poise to 100 poise.
3. A process as specified in claim 1 wherein the liquid melt has a
viscosity which is within the range of 0.1 poise to 10 poise.
4. A process for removing foreign materials from a polyethylene
terephthalate feed stream which comprises: (1) depolymerizing the
polyethylene terephthalate feed stream into a liquid melt having a
viscosity which is within the range of 0.001 poise to 1000 poise, (2)
feeding the liquid melt into a separation device, (3) allowing low density
foreign materials to migrate to the surface of the liquid melt, and (4)
removing depolymerized polyethylene terephthalate from below the surface
of the liquid melt.
5. A process as specified in claim 4 wherein the separation device is
enhanced by centrifugal action.
6. A process as specified in claim 1 wherein the low density foreign
materials migrate to the surface of the liquid by flotation and wherein
the high density foreign materials migrate to the bottom of the liquid by
sinking.
7. A process as specified in claim 1 wherein the depolymerized polyethylene
terephthalate is polyethylene terephthalate oligomer.
8. A process as specified in claim 1 wherein the melt is at a temperature
which is within the range of about 180.degree. C. to about 310.degree. C.
Description
BACKGROUND OF THE INVENTION
In 1989 approximately 700 million pounds of polyethylene terephthalate
(PET) were consumed to produce soft drink bottles. Presently about 28% of
this material is recycled with the remainder being placed in landfills or
incinerated. Due to problems with overloaded landfills and the negative
environmental image plastics has received recently, bills have been
introduced aimed at establishing bottle deposits and the number of
curbside recycling programs has increased tremendously. The collection of
PET bottles and food trays through these programs has resulted in a source
of post consumer PET which has been used for a number of applications.
Generally these applications involve the conversion and fabrication of the
post-consumer PET into materials of lower value. Examples of such
applications are polyols for unsaturated polyesters or polyurethanes,
fiberfill, carpet fibers, and strapping. Recycled PET is also blended with
other materials such as polybutylene terephthalate, polycarbonate, or
glass fibers, etc., for automotive as well as other engineering
applications.
Post-consumer PET can also be recycled into resin which can be used in
manufacturing containers for foods and beverages, such as carbonated
beverage bottles. In such procedures, the post-consumer PET is generally
depolymerized to oligomers or its monomers which are subsequently utilized
as a raw material in the preparation of the recycled PET resin. Such a
procedure for depolymerizing PET is described in U.S. Pat. No. 3,703,488
and U.S. Pat. No. 3,884,850.
In such recycling programs, it is important to separate the post-consumer
PET from other plastics in the recycle stream. Foreign materials can be
removed from such post-consumer PET feed stream by hand separation.
Foreign materials, such as other plastics, can also be removed from the
post-consumer PET feed stream by flotation procedures which are based upon
density and/or wettability differences between the plastics. Nevertheless,
such procedures generally do not result in the total removal of foreign
materials from the post consumer PET feed stream. This is particularly
true of glues, small particle size foreign materials, and polymeric melt
blends. For instance, some post-consumer PET sources are melt blends of
the PET with one or more other plastics. In the case of such melt blends,
the other plastics cannot be removed from the PET by physical separation
procedures. For instance, dual ovenable trays which are used extensively
by the frozen prepared food industry typically contain about 97% PET and
about 3% linear low density polyethylene. It is, of course, not possible
to separate the polyethylene from the PET in such melt blends by
mechanical means. Nevertheless, there is a tremendous need to remove
polyethylene and other foreign materials which are intimately mixed with
the PET from such feed streams.
SUMMARY OF THE INVENTION
By utilizing the techniques of this invention, foreign materials which are
in PET feed streams can be removed. This technique is particularly useful
in the removal of polyethylene and other polymeric material which have
been melt blended with the PET in the post-consumer feed stream. By
utilizing this procedure, metals, such as residual catalysts, can be
removed from PET feed streams. This technique involves depolymerizing the
PET in the feed stream and subsequently separating the foreign materials
therein from the PET melt.
The subject invention more specifically discloses a process for removing
foreign materials from a polyethylene terephthalate feed stream which
comprises: (1) depolymerizing the polyethylene terephthalate feed stream
into a liquid having a viscosity which is within the range of 0.001 poise
to 1000 poise, (2) feeding the liquid into a separation device, (3)
allowing low density foreign materials to migrate to the surface of the
liquid, (4) allowing high density foreign materials to migrate to the
bottom of the liquid, and (5) removing depolymerized polyethylene
terephthalate from an area which is located intermediately between the
surface of the liquid and the bottom of the liquid.
The present invention also reveals a process for removing foreign materials
from a polyethylene terephthalate feed stream which comprises: (1)
depolymerizing the polyethylene terephthalate feed stream into a liquid
having a viscosity which is within the range of 0.001 poise to 1000 poise,
(2) feeding the liquid into a separation device, (3) allowing low density
foreign materials to migrate to the surface of the liquid, and (4)
removing depolymerized polyethylene terephthalate from below the surface
of the liquid.
The subject invention further discloses a process for removing foreign
materials from a polyethylene terephthalate feed stream which comprises:
(1) dissolving the polyethylene terephthalate feed stream in a sufficient
amount of a solvent to produce a liquid system having a viscosity which is
within the range of 0.001 poise to 1,000 poise, (2) feeding the liquid
system into a separation device, (3) allowing low density foreign
materials to migrate to the surface of the liquid system, and (4) removing
dissolved purified polyethylene terephthalate from below the surface of
the liquid system.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a prospective, fragmentary, and diagrammatical view of a
separation device which can be utilized in the process of this invention.
DETAILED DESCRIPTION OF THE INVENTION
The post-consumer PET feed streams purified by the process of this
invention contain PET and various foreign material. The foreign materials
present in such post-consumer PET feed streams can include glues,
elastomers, linear low density polyethylene, high density polyethylene,
polypropylene, ethyl vinyl acetates, polyvinyl chloride, residual
catalysts, and the like. It is very desirable for as much foreign material
to be removed from the PET feed stream as possible by mechanical or other
physical means before implementing the process of this invention. In other
words, the PET feed stream should be as pure as possible with the
technique of this invention being an additional purification step for
removing foreign materials in the recycling operation.
The PET in the post-consumer recycle stream is typically comprised of
repeat units which are derived from terephthalic acid or a diester thereof
and ethylene glycol. However, it is understood that the PET can also be
modified with small amounts of other monomers. Such modified PET can
contain small amounts of repeat units which are derived from diacids other
than terephthalic acid and/or glycols in addition to ethylene glycol. For
instance, small amounts of isophthalic acid or a naphthalene dicarboxylic
acid can be used in the diacid component utilized in preparing the PET.
PET which has been modified with a small amount of a diol containing from
3 to 8 carbon atoms is also representative of such a modified PET. For
instance, a small amount of 1,4-butane diol can be utilized in the glycol
component used in preparing the modified PET. Normally, no more than about
5 weight percent of the repeat units in such modified PET will be
comprised of diacids or diols other than a terephthalic acid and ethylene
glycol. It is, of course, contemplated that diesters of such dicarboxylic
acids and diols can also be used. In most cases, such modified PET will
contain less than about 3% diacids other than terephthalic acid and less
than 3% diols other than ethylene glycol. Such modified polyesters most
typically contain only about 1% dicarboxylic acids other than terephthalic
acid and/or less than 1% glycols other than ethylene glycol. Polyethylene
isophthalate and copolymers thereof will also normally be in the recycle
feed stream and are considered to be PET for the purposes of this
invention.
The first step in the practice of this invention involves depolymerizing
the PET in the post-consumer PET feed stream to a viscosity which is
within the range of 0.001 poise to 1,000 poise. In an alternative
embodiment of this invention, the post-consumer PET feed stream can be
dissolved in a solvent to produce a liquid system. For ease of separation,
it is desirable for this viscosity to be as low as possible.
Standard depolymerization techniques can be used in this step. For
instance, the procedures described in U.S. Pat. No. 3,703,488 and U.S.
Pat. No. 3,884,850, the teachings of which are incorporated herein by
reference in their entirety can be used in depolymerizing the PET in the
post-consumer PET feed streams. It is generally desirable to depolymerize
the PET by adding from about 2 weight percent to about 50 weight percent
ethylene glycol to the PET feed stream and heating the PET/ethylene glycol
mixture at a temperature within the range of about 180.degree. C. to about
310.degree. C. to cause depolymerization. It is preferred to use a
temperature within the range of 220.degree. C. to 270.degree. C. It is
generally preferred to use from about 7 weight percent to about 20 weight
percent ethylene glycol. Depolymerization procedures which use water, acid
or caustic in place of ethylene glycol can also be used. In any case, the
depolymerization will be conducted until the viscosity is reduced to
within the range of about 0.001 poise to about 1,000 poise. It is
preferred for the depolymerization to be carried out to the extent that
the viscosity of the liquid has been reduced to within the range of 0.001
poise to 100 poise. It is more preferred for the liquid to have a
viscosity which is within the range of 0.1 to 10 poise.
In an alternative embodiment of this invention, the first step can be
carried out by dissolving the polyethylene terephthalate feed stream in a
sufficient amount of an organic solvent to produce a liquid system having
a viscosity which is within the range of 0.001 poise to 1,000 poise. A
wide variety of solvents or solvent systems capable of dissolving the PET
at room temperature or at a slightly elevated temperature can be employed.
However, it is important to select a solvent which is not capable of
dissolving the foreign materials in the feed stream. It is preferred for
the solvent used to have a low boiling point so that residual solvent can
be more easily removed from the PET which is recovered. Some
representative examples of suitable solvents include nitro-benzene,
acetonapthone, hexafluoroacetone, hexafluoroisopropanol, meta-cresol,
trifluoroacetic acid/methylene chloride mixed solvent systems, nitro.
benzene/tetrachloroethane mixed solvent systems,
hexafluoroisopropanol/chloroform mixed solvent systems, and
tetrachloroethane/phenol mixed solvent systems. Trifluoroacetic
acid/methylene chloride mixed solvent systems which contain from about 25
weight percent to about 75 weight percent trifluoroacetic acid and from
about 25 weight percent to about 75 weight percent methylene chloride are
preferred. It is generally more preferred for such solvent systems to
contain from about 40 to about 60 weight percent tetrafluoroacetic acid
and from about 40 to about 60 weight percent methylene chloride.
The liquid, which is generally a melt, produced in the first step is then
fed into a separation device. This separation device as depicted in FIG. 1
acts as a float cell which separates low density foreign materials and
high density foreign materials from the PET oligomer produced by the
depolymerization step. This invention is based upon the unexpected
discovery that most common polymers which are found in the PET feed stream
as contaminants are immiscible with the liquid depolymerized PET and will
float upon its surface. This causes the low density foreign materials to
migrate to the surface of the melt by flotation. Techniques which rely on
migration by centrifugal force or centripetal force can also be used.
Heavy foreign materials, such as stones, glass, fibers, and metals, will
sink to the bottom of the liquid depolymerized PET.
A separation device which is designed for use in practicing the subject
invention is shown in FIG. 1. This separation device is essentially a
closed vessel 1. The depolymerized PET made in the depolymerization step
is fed into the separation device through inlet pipe 2. The melt
discharged through the inlet pipe 2 flows down trough 3 to the surface of
the melt 9 in the separation device. The trough 3 allows the melt from the
depolymerization step to be gently discharged onto the surface of the melt
9 regardless of the level of the melt in the separation device. This
prevents low density foreign materials from plunging deep into the melt.
Accordingly, the low density foreign materials simply remain on the
surface of the melt without needing to float to the surface of the melt 9
from areas far below the surface of the melt 9. In the separation device
shown in FIG. 1, the trough 3 simply spirals around the inside wall of the
closed vessel 1. However, it is contemplated that other trough designs
which will accomplish the same purpose can be utilized. Nevertheless,
there is some benefit to the spiral design which causes some degree of
spinning action and some centrifugal force which facilitates the
separating action.
The depolymerized PET in the liquid is removed from an area which is
located intermediately between the surface of the melt 9 and the bottom of
the liquid (the bottom of the closed vessel). Of course, the low density
foreign materials accumulate on the surface of the liquid with the high
density foreign materials sinking to the bottom of the liquid. Thus, the
portion of the melt which is located intermediately between the surface of
the liquid and the bottom of the liquid contains mostly depolymerized PET
of reasonably high purity. The depolymerized PET is removed from this area
located intermediately between the surface of the liquid and the bottom of
the liquid through depolymerized PET discharge pipe 4. It is preferred for
the inlet 10 to PET oligomer discharge pipe 4 to be surrounded by a stand
pipe 6 which prevents low density foreign materials and high density
foreign materials from being sucked directly into the PET oligomer
discharge pipe 4. The inner diameter of shield pipe 6 is sufficiently
greater than the outside diameter of depolymerized PET discharge pipe 4 to
allow for the smooth flow of depolymerized PET between the two pipes
allowing for the depolymerized PET to be withdrawn through the inlet 10 to
the depolymerized PET discharge pipe 4. Stand pipe 6 which operates with
discharge pipe 4 as underflow and overflow weirs preventing low density
material and/or high density material from being sucked into discharge
pipe 4. The stand pipe 6 will include at least one hole 7 in it at a level
which is above the maximum operating surface of the liquid. Such a hole 7
allows the pressure, both inside the stand pipe 6 and outside of the stand
pipe but within the closed vessel 1, to be at equilibrium.
High density foreign materials which sink to the bottom of the liquid in
the closed vessel can be continuously or periodically removed through high
density foreign material discharge pipe 5. Heavy contaminants such as
stones, glass, and metals which travel down the entire length of the
trough 3 can be collected and removed from the separation device through
heavy contaminant discharge pipe 8. In operating the separation device, it
will be necessary to remove the low density foreign material floating on
the surface of the liquid from time to time or continuously. This can be
accomplished by discharging the entire contents of the closed vessel
through the high density foreign material discharge pipe 5. In doing so,
all of the low density foreign material floating on the surface of the
liquid can be removed. The frequency at which such a discharge step is
needed will depend upon the purity of the post-consumer PET feed stream
and the corresponding amount of low density foreign materials therein.
This invention is further illustrated by the following examples which are
merely for the purpose of illustration and are not intended to limit the
scope of the invention or the manner in which it can be practiced. Unless
otherwise indicated herein all parts and percentages are given by weight.
EXAMPLE 1
In this experiment, the process of this invention was utilized to show that
melt blended linear low density polyethylene could be removed from PET.
This procedure was carried out utilizing laboratory equipment. In the
procedure used, 500 grams of depolymerized PET from clear post consumer
bottles was melted in a flask at a temperature of about 220.degree. C. The
depolymerized PET had an average chain link of four repeat units. After
the depolymerized PET had completely melted, the temperature was increased
to about 255.degree. C. Then, 100 grams of sheet which was comprised of a
melt blend containing about 97% PET and about 3% linear low density
polyethylene (by weight) was added to the flask in small strips over a
period of about 20 minutes. The polyethylene in the sheet was dispersed to
a particle size of less than 1 micron. The polyethylene rose to the
surface of the melt and was pushed to the center due to the centrifugal
action caused by the stirrer. This is due to the fact that the
polyethylene is lighter than the PET melt. The specific gravity of the
linear low density polyethylene was about 0.97 and the specific gravity of
the PET melt was about 1.22. After about 3 grams of the sheet had melted,
a small amount of polyethylene rose to the surface and began to
agglomerate and began to cling to a stirring rod in the flask at which
point about 3 ml of ethylene glycol was added. The melt was maintained at
a temperature of about 255.degree. C. for about 20 minutes to ensure that
all of the PET from the sheet was depolymerized and to allow sufficient
time for all of the polyethylene to rise to the surface. It was observed
that the polyethylene particles agglomerated together as they rose to the
surface forming large clumps which were thrown to the center of the flask
due to the centripetal action. The polyethylene accordingly agglomerated
around the stirrer located in the center of the flask. The polyethylene
which had agglomerated was removed from the melt by simply pulling the
stirrer from the flask. This was possible because the polyethylene adhered
together as a solid mass on the surface of the stirrer.
After the polyethylene was removed from the stirrer, the stirrer was,
again, inserted into the flask. After stirring had been resumed, a second
batch of sheet was added to the flask with about 5 ml of ethylene glycol.
The separation continued as before. Again, the polyethylene agglomerated
and was collected from the stirrer. The stirrer was, again, cleaned and
replaced in the flask. After stirring was resumed, a third hundred gram
batch of sheet was added to the flask with an additional 5 ml of ethylene
glycol.
The total quantity of polyethylene collected in this experiment was 9
grams. This represents a polyethylene removal efficiency of 100%. This
experiment demonstrates the operability of the subject invention and shows
that it is 100% efficient.
EXAMPLE 2
This experiment shows that the technique of this invention can be utilized
in large scale operations. In the procedure used, 1,644 lbs. (746 kg) of
depolymerized PET containing 0.36 weight percent polyethylene was fed into
a separation device having the design shown in FIG. 1. The separation
device was maintained at a temperature above the melting point of the
depolymerized PET. The polyethylene floated to the surface of the melt and
agglomerated into a solid mass. The purified stream of depolymerized PET
was recovered from the separation device and was determined by DSC
(differential scanning calorimetry) to be free of polyethylene. In fact,
the depolymerized PET feed stream was used in the synthesis of PET bottle
resin. Bottles were made from such resin with no evidence of polyethylene
contamination being observed. This illustrates the efficiency of the
separation device and the process of the subject invention.
Variations in the present invention are possible in light of the
descriptions of it provided herein. It is, therefore, to be understood
that changes can be made in the particular embodiments described which
will be within the full intended scope of the invention as defined by the
following appended claims.
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